Sensors are used in automotive and various other applications for a variety of purposes, such as sensing fluid or air pressure within an automotive fuel system and sensing motion as part of a passenger passive restraint system. A third type of sensor finding use in automotive applications is flow sensors, which serve such purposes as determining the flow rate of intake air to an engine. On the basis of reliability and cost, a trend in the automotive industry is to fabricate such sensors in the form of monolithic semiconductor sensors that are micromachined from silicon wafers.
The prior art has generally relied on flow sensors in the form of hot wire devices and pressure sensing diaphragms modified to detect air flow. Hot wire devices are generally polysilicon or metal runners formed on a silicon chip and indicate fluid flow by sensing the heat removed from the hot wire by the fluid, while pressure sensing diaphragms rely on a venturi effect as a fluid passes through an opening in the diaphragm to deflect the diaphragm, which can then be sensed by piezoresistive or capacitive techniques. While such flow sensor designs have found acceptance in the industry, their fabrication, size and complicated construction diminish their desirability and potentially their ruggedness for use in automotive applications. Such shortcomings can be exasperated by the presence of signal conditioning or processing circuitry on the same structure that supports the sensing element. Furthermore, these prior art sensor designs can be impractical for use in sensing flow in applications where the direction of fluid flow is not unidirectional, i.e., the fluid may reverse its direction of flow.
Therefore, it would be desirable to provide a semiconductor flow sensor that is relatively uncomplicated in its fabrication and construction, can be fabricated to have a monolithic structure on which signal conditioning and processing circuitry can be provided, and is characterized by an efficient use of material so as to minimize the size of the sensor. It would particularly be desirable if the sensitivity of such a flow sensor could be readily modified during fabrication in order to optimize the sensor for the conditions in which in will be used.